Effect of Solar Wind Electrons on Stimulating the Oxidation and Hydroxylation of Lunar Materials-I: Experimental Simulation on Fe Metal
Date:
Hosts: Roger Wiens and DJ Lee
Abstract: Orbital observations by the Moon Mineralogy Mapper (M3) have identified hematite (Fe2O3) and FeOOH in the high-latitude regions of the Moon. Hematite was also identified in the returned Chang’E-6 sample. Fe3+-bearing oxide and hydroxide formation is a conundrum because of the overall highly reduced lunar surface environment. The occurrence of Fe oxides and hydroxides with the lunar high-latitude regions is relevant to space weathering of polar H2O ice deposits. In this study, we simulate the effects of solar wind electrons using energetic electron impact (EEI) on two forms of pure iron metal in two environmentally controlled vacuum chambers and characterize the reaction products. Our experimental observations demonstrate that EEI activated the surface of Fe metal and, in the presence of trace amounts of H2O, accelerated the oxidation and hydroxylation of Fe to form magnetite (Fe3O4), hematite (Fe2O3), and FeOOH at grain surfaces after tens of hours. In contrast, a simple coexistence of Fe metal with water, even with added O2 and slightly raised temperatures after tens of days, fails to form hematite or FeOOH. We infer that EEI-induced plasma-material interactions, specifically electron-stimulated desorption and dissociation of water molecules, allow activated oxygen (OI*) and OH* to enter and alter the surface structure and composition of the Fe-metal, stimulating the growth of Fe oxides and oxyhydroxides. These results provide a viable mechanism for the formation of hematite and FeOOH revealed by orbital remote sensing at the high latitude regions of the Moon.